1. Field of the Invention
The invention relates to the field of the atomic clocks, and more particularly to a physical system for a chip-scale coherent population trapping (CPT) atomic clock.
2. Description of the Related Art
Atomic clock is a tool that, to measure time, utilizes quantum transition between different energy levels in the microcosm of an atom.
The atomic clock provides frequency signals of high stability and high accuracy, which satisfy multiple requirements on timing and punctuality, navigation, precision measurement, and high-speed communication.
A physical system is a core part of the atomic clock. The quality of the physical system directly determines performances and indexes of the atomic clock. Thus, the design of the physical system is one of the key factors in making the atomic clock.
In a conventional physical system, an alkali metal atom and buffer gas in a bubble are heated to a temperature higher than the room temperature to produce alkali metal atom vapor. The buffer gas is an inactive gas such as nitrogen, methane, helium, or a mixture thereof. The buffer gas is used to narrow the line width, quench the fluorescence, and mix the energy levels. The alkali metal atom is cesium 133, rubidium 87, or rubidium 85.
Resonance between the ground-state hyperfine sublevels of the alkali metal atom is used to discriminate the frequency of the injected microwave. With reference to FIG. 1, because the two energy levels of mF=0 (“0-0” energy levels) are not sensitive to a weak magnetic field, the transition frequency υ00 therebetween is generally used as a discrimination frequency of the atomic clock. When the microwave frequency is scanned through the transition frequency, the resonance signal represents a concave or a convex in the detected optical signal. A local oscillator is utilized to produce the microwave, and the microwave is locked in a center transition frequency corresponding to the concave or the convex of the resonance signal, thereby obtaining a precise clock signal output of the local oscillator.
Coherent population trapping (CPT) atomic clock is a more recent type of atomic clock that features small volume, low energy consumption, and fast start, and has broad application prospects. Principally, the CPT atomic clock is so far the only miniaturized atomic clock. A chip-scale CPT atomic clock can be realized by using the micro-electro-mechanical systems (MEMS) process to manufacture a quantum system and an application specific integrated circuit (ASIC) process to manufacture an electronics system.
A schematic diagram of a physical system of a passive bubble-type CPT atomic clock is shown in FIG. 2. A microwave of a certain power passes through a capacitor and is mixed into a DC current. The mixed electric signal is injected into a vertical-cavity surface-emitting laser (VCSEL) to produce multi-sideband of light, and a fundamental frequency of the multi-sideband of the light is controlled by the injected DC current and a temperature of a laser. The frequency difference between adjacent sidebands is equal to a microwave frequency. An intensity of each sideband of the light satisfies a Bessel function. CPT resonance is excited by the two sidebands.
If the ±1st order sidebands of the light excite the CPT resonance, it is called a half-width modulated excitation. If the fundamental frequency (0 order) and +1st (or −1st) order sidebands of the light excite the CPT resonance, it is called a full-width modulated excitation.
A quarter wave plate ((λ/4 wave plate) functions in converting a linearly polarized light output by the VCSEL into a left-handed (σ−) or right-handed (σ+) circularly polarized light. The bubble is disposed in an environment where the temperature can be accurately controlled, the atom and the laser interact with each other so that an atom vapor is produced.
On the premise that the environmental magnetic field is shielded, solenoids (not shown in FIG. 2) arranged outside the bubble produce a magnetic field in parallel to a transmission direction of the light. A photo detector detects the laser passing through the bubble and converts the laser into a photocurrent signal. The injected DC current and the temperature of the laser tube are properly controlled such that the two CPT exciting sidebands of the light output by the VCSEL excite electric dipole transitions (D1 line or D2 line transition) from two ground states to the same excited states. The microwave frequency is regulated to change the frequency difference between the two sidebands of the light. When the frequency difference is scanned through the resonance frequency of the hyperfine energy levels “0-0”, a resonance signal emerges in the photocurrent output by the photo detector. The resonance signal is processed and a signal for feedback control of the microwave frequency is obtained. After a closed-loop control, a precise clock signal output of the local oscillator is obtained.
In the above-described conventional CPT atomic clock a single σ− or σ+ circularly polarized laser serves as the light source to excite atom transitions. However, such a scheme is disadvantageous in that the atom is prone to be accumulated at ground-state magnetic sublevels having a minimum (or maximum) magnetic quantum due to an optical pumping effect of the circularly polarized laser on magnetic hyperfine splitting atomic sublevels. Such magnetic sublevels have no contribution to the CPT dark state, thereby being called polarized dark state. The phenomenon is illustrated in FIG. 1. Thus, the signal contrast (a ratio between an amplitude of the photocurrent signal increased by CPT resonance and an amplitude of the photocurrent signal of non-resonance) is not high and the stability of the CPT atomic clock is not high.
The above polarized black state can be eliminated by adopting a scheme including exciting the CPT resonance by orthogonal circularly polarization and using the σ− and σ+ circularly polarized light to interact with the atom, so that a CPT signal having high contrast is obtained. The scheme can be illustrated in a four-energy-level drawing showing interaction between the light and the atom, as shown in FIG. 3. A phase difference between the σ− and σ+ circularly polarized light is regulated to be (2n+1)π (n represents an integer) by time or space delay, and the in-phase superposition effect of the state function is achieved, so that the CPT resonance signal is enhanced and the atom is centralized at the “0-0 energy level”, as shown in FIG. 4. Thus, the short term stability of the atomic clock manufactured by using the scheme of the CPT resonance excitation by orthogonal circularly polarization is higher than the conventional CPT atomic clock.